Nick Clegg will send an austere message to his party on Wednesday, telling them the Liberal Democrats are willing to impose several more years of spending restraint, are now anchored in the centre ground and are determined never to revert to being a party of protest and opposition.

But Clegg's efforts to persuade his party of the virtues of coalition compromise in his keynote speech were dealt a blow when conference delegates, appealing to the party's long liberal traditions, spurned the front bench and voted overwhelmingly on Tuesday to reject the coalition's plans for secret courts.

The vote will, at the minimum, require Clegg to go back to the Home Office and intelligence services to seek fresh concessions to the justice and security bill. The conference voted for the entire section of the bill introducing secret courts, or closed material procedure, to be scrapped. It is currently in the Lords and has already come under attack from senior lawyers.

The defeat represented an awkward overture to Clegg's main conference speech, especially a long passage in which he is expected to hail the way the party had confounded received wisdom by showing it was capable of making the transition from opposition to the discipline of government.

Clegg will try to persuade the electorate the party has changed irretrievably, saying: "The past is gone and it isn't coming back. If voters want a party of opposition – a 'stop the world I want to get off' party – they have got plenty of options, but we are not one of them."

He will say the party only has a meaningful future as one of the three parties of government, not as a third party.

He will admit he knows there are "some in the party who faced with several more years of spending restraint would rather turn back, than press on, break our deal with the Conservatives, give up on the coalition and present ourselves to the electorate in 2015 as a party unchanged".

He will acknowledge this may be an alluring prospect for some, but "gone would be the difficult choices, the hard decisions the necessary compromises".

He will declare: "The choice between the party we were and the party we are becoming is a false one. The past is gone and is not coming back" .

The leadership's aim is to try to bookend Clegg's week by starting with the ground-clearing apology for making the fateful populist promise on tuition fees, and then conclude it with a vision of a party now capable of taking the difficult, painful decisions. Clegg will highlight the decisions to reduce the deficit by a quarter saying it has laid the foundations for a stronger and more balanced economy.

He will also explicitly link his own party's fortunes to the fate of the economy saying the "success of each depends on the success of the other".

Clegg will also use his speech to announce £50m in catch-up classes to help children who have underperformed in English and maths at Key Stage 2.

He will claim the measure goes to the root of the party's philosophical core. He defines this as "the freedom to be who you are. The opportunity to be who you could be. That is essence is the Liberal promise".

Clegg's leadership has been unchallenged at this conference but there are fears that such an explicit centre ground strategy may unnecessarily endanger seats at the next election. It causes private unease among some of his cabinet colleagues if it also entails, as Clegg sometimes suggests, permanently giving up on leftwing former Lib Dem voters who have switched to Labour.

The cabinet members believe that in many Liberal Democrat seats, an anti-Tory tactical vote can still be reactivated, and it would be foolish to spurn such voters in search of a new centre ground.

There was also pressure on Clegg to take a tougher line on so-called middle class welfare, amid fears that further chunks will be taken out of low income welfare before the 2015 election. Don Foster, the communities minister, urged David Cameron to break his pledge not to withdraw fuel allowance from well- off pensioners. "If we can get something like this implemented now, then that would be better."

But Clegg said there will be no change in this parliament: "On things such as winter fuel allowance, free bus passes, and free TV licences, there is of course a debate to have in the next parliament, because we've ruled it out in this coalition agreement … there's a question mark on whether people such as Peter Stringfellow or Alan Sugar have available to them free bus passes, subsidised by normal workers paying normal taxes and struggling to make ends meet."

Sugar retorted in a tweet: "Even if I did have a bus pass I've personally paid £10sM tax, my companies 100s of millions in the past 45 years. What has Clegg done ?"